The scanning transmission electron microscope (STEM) has made significant contributions to structural biology by providing accurate determinations of the molecular masses of large protein assemblies that have arbitrary shapes and sizes. Nevertheless, STEM mass mapping has been implemented in very few laboratories, most of which have employed cold-field emission gun (FEG) electron sources operating at acceleration voltages of 100 kV and lower. Here we show that a 300 kV commercial transmission electron microscope (TEM) equipped with a thermally assisted Shottky FEG can provide accurate STEM mass measurements of nanoparticles that have potential for use in cancer theranostics. Polyamidoamines (PAMAMs) comprise a common class of dendrimers that are highly soluble in aqueous solution and can contain a high number of chemically active terminal amino groups. In particular, PAMAM dendrimers conjugated with Gd3+-diethylene triamine pentaacetic acid (DTPA) could provide useful magnetic resonance (MR) imaging contrast agents for neoplastic tissue. The effectiveness in the use of dendrimers for medical imaging and drug delivery applications can depend, among other factors, on their average diameter, average molecular weight, degree of conjugation with chelating agents, number of Gd atoms, and degree of monodispersity. For MR imaging and treatment of neoplastic tissue, for example, these variables can be expected to impact critical physiological properties of the dendrimers such as blood-circulation half-life, vascular permeability, and retention time within the tissue. Native, unconjugated dendrimers typically have well-defined molecular weights and high degree of monodispersity. However, after conjugation with Gd-DTPA chelates, some of the physical properties of the native dendrimers become less well-defined and are difficult to accurately quantify through direct measurements. We have used a combination of STEM mass mapping and energy-filtered transmission electron microscopy (EFTEM) to characterize specific generations (G5, G6, G7) of PAMAM dendrimers. STEM images provide the distribution of molecular weights of the diffferent dendrimer generations, whereas EFTEM imaging at the Gd N45 excitation edge provides the numbers of bound Gd atoms. This approach provides a quantitative assessment of the degree of monodispersity of nanoparticles, which will be useful in developing these reagents for eventual applications in nanomedicine.